How to print a single blank check from my bank amount??? (sounds sketchy but isn't!)

This Friday, I have to pay for a medical appointment with a check. I do not know the exact amount that I'll have to pay (the doc has a sliding scale), but it'll be <$300. I have enough money in my checking account to pay for this, but I don't actually own a checkbook (I pay my rent and things with online banking). I don't want to order a whole checkbook because I'm moving to another city in about a month (not to mention the fact that overnight shipping for checks costs like $25, which is dumb), and my stupid bank will only give me a check or money order if I know the amount. I've looked up check printing and stuff online and it looks like Walmart has this service... but it seems like they can't do it in the store, and are only able to do it online? is there a company that can do this???

tl;dr: is there a cheap and immediate way to print a single blank check that will draw from my checking account?